In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique of transcribing a fine pattern on a Si substrate or the like, which is required for forming an integrated circuit having such fine pattern. However, the microsizing of semiconductor devices has been accelerated, and on the other hand, the conventional photolithography method approaches the limit. In the photolithography method, it is said that the resolution limit for a pattern is about ½ of exposure wavelength and is about ¼ of exposure wavelength even if an immersion method is used. Even though the immersion method with an ArF laser (193 nm) is used, it is estimated that the limit is about 45 nm. Now, as a new technique of light exposure using a wavelength of 45 nm or shorter, EUV lithography which is an exposure technique using EUV light having a further shorter wavelength than ArF laser has been considered to be promising as an exposure technique for 45 nm or below. In this description, the EUV light indicates a ray having a wavelength in a soft X-ray region or a vacuum ultraviolet ray region, specifically, a ray having a wavelength of about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
The EUV light is apt to be absorbed by any substance and the refractive indices of the substance with respect to this wavelength is close to 1, and therefore, the conventional dioptric system such as the photolithography using visible light or ultraviolet light cannot be employed. For this reason, a catoptric system, i.e. a reflective photomask and a mirror are employed in the EUV light lithography.
A mask blank is a laminated member before patterning which is used for producing a photomask. The EUV mask blank has a reflective layer for reflecting EUV light and an absorbing layer for absorbing EUV light which are formed in this order on a substrate such as glass or the like. For the reflective layer, normally used is a multilayered reflective film which is formed by laminating alternately a layer of high refractive index and a layer of low refractive index whereby the reflectance of light is increased when EUV light is irradiated to the layer surface. As the absorbing layer, a material having a high absorbing coefficient to EUV light, specifically, a material having Ta or Cr as major component, is used.
On the absorbing layer of the EUV mask blank, a low-reflective layer with respect to a mask pattern inspection light is normally formed. In order to detect the presence or absence of a defect of pattern after the formation of a mask pattern, a ray which has the wavelength region of deep ultraviolet light (190 to 260 nm) is employed. The inspection of pattern employing the ray having the above-mentioned wavelength region is to detect the difference of reflectivity between an area where the low-reflective layer and the absorbing layer have been removed by a patterning process and an area where the low-reflective layer and the absorbing layer remain, namely, to detect the contrast of reflection light at the surfaces of these areas whereby the presence or absence of the defect of pattern is examined. In order to increase the sensitivity of the detection of mask pattern, the contrast should be increased. For this purpose, it is normally required that the low-reflective layer has a low reflectivity with respect to the above-mentioned wavelength region, namely, the reflectivity with respect to the wavelength region is 15% or less.
Patent Document 1 describes that a low-reflective layer comprising an oxide of tantalum-boron alloy (TaBO) or an oxynitride of tantalum-boron alloy (TaBNO), which is formed on an absorbing layer comprising a nitride of tantalum-boron alloy (TaBN), has preferably a low reflectivity with respect to the wavelength region (190 nm to 260 nm) of mask pattern inspection light.
Patent Document 2 describes that it is preferred to form a low-reflective layer comprising metal, silicon (Si), oxygen (O) and nitrogen (N) on an absorbing layer in order to adjust the reflectivity with respect to the wavelength region (190 nm to 260 nm) of mask pattern inspection light.